PURPOSE: To enable display of a coordinate in a present position, being under machining, irrespective of a display picture through simple control, by a method wherein a window on which a coordinate value is displayed can be displayed, moved, and erased on the screen of a machining data display means.

CONSTITUTION: During machining, machining data is written in a screen buffer 6 from a screen display table 5 through a window control circuit 11, and is displayed on a CRT 7. Meanwhile, a coordinate X . Y in a position, being under present machining, from a detector 1 is stored in a coordinate value table 4 through a theoretical circuit 3. In which case, when window display is instructed by means of an input key 2, the theoretical circuit 3 reads a coordinate value from the coordinate table 4, feeds machining date in a window region 8 from the screen buffer 6 to a window buffer 10 through a window display data table 9, a control device 11, and the window buffer 10. After the theoretical circuit protects a screen data 22, a coordinate value in a present position is displayed in the window region of the CRT 7. This constitution enables improvement of machining efficiency.